On 11/11/2021 08:02, Sebastian Huber wrote:> On 09/11/2021 13:06,
Sebastian Huber wrote:
On 09/11/2021 08:50, Sebastian Huber wrote:
On 09/11/2021 08:41, Chris Johns wrote:
We could also use something like this:
static inline struct timespec rtems_clock_get_realtime(void)
{
struct timespec time_snapshot;
_Timecounter_Nanotime( &time_snapshot );
return time_snapshot;
}
Unfortunately GCC is not able to optimize this.
Ah OK. This can be fixed and the performance improved but once the
API is set it
cannot change or do you think we can add a check later and not break
the API?
I filed a GCC bug for this:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103150
It seems I was not the only one noticing issues related to structure
returns:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101926
However, if we want a foolproof API, then I would prefer the
structure return over the return status and pointer argument.
Compilers may get better in the future. clang has similar issues, so
this is not only a GCC problem.
We have at least three options for the API:
1. Use the existing FreeBSD implementation as is:
void rtems_clock_get_realtime(struct timespec *);
This is the easiest and most efficient approach.
2. Check for NULL and return a status:
rtems_status_code rtems_clock_get_realtime(struct timespec *);
This requires a wrapper function which is a bit less efficient and
needs more code/testing:
rtems_status_code
rtems_clock_get_realtime(struct timespec *time_snapshot)
{
if ( time_snapshot == NULL ) {
return RTEMS_INVALID_ADDRESS;
}
_Timecounter_Nanotime( time_snapshot );
return RTEMS_SUCCESSFUL;
}
3. Return the structure and use the existing implementation:
static inline struct timespec rtems_clock_get_realtime(void)
{
struct timespec time_snapshot;
_Timecounter_Nanotime( &time_snapshot );
return time_snapshot;
}
This is currently not well supported by existing compilers:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103150
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101926
4. Do nothing for a NULL pointer:
void rtems_clock_get_realtime(struct timespec *);
This requires a wrapper function which can use a tail call optimization:
void
rtems_clock_get_realtime(struct timespec *time_snapshot)
{
if ( time_snapshot == NULL ) {
return;
}
_Timecounter_Nanotime( time_snapshot );
}
How do we want to proceed with this? We ship the high performance and
very useful clock routines from FreeBSD since 2015. I just would like to
add an RTEMS signature for them and document them in the Clock Manager.
Currently, these routines are the most efficient way to get clock values
in RTEMS. Developers afraid of unchecked NULL pointers may use existing
RTEMS directives or clock_get(). It would be nice if we can decide on a
way forward.
--
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